The present invention relates to cardiac defibrillation, and more particularly to a defibrillation system including a pulse generator which generates a long duration defibrillation waveform.
When used hereinafter, the term"defibrillation" is meant to include high energy defibrillation and lower energy cardioversion.
Present defibrillation devices generate and deliver a truncated exponential waveform with a duration less than 12 msec, or some variation thereof. According to conventional wisdom, the use of defibrillation waveforms with durations 15-20 msec or longer would be counter-indicated for defibrillation. In particular, it has been the assumption by experts in the field that the strength-duration relationship of a biphasic waveform are the same as for a monophasic waveform. That is, it is the general consensus that increasing waveform duration prohibitively increases the necessary energy requirements.
In the last five years, there have been several reports of human and animal studies showing that some biphasic waveforms are more efficacious than monophasic waveforms. See Jones et al., "Increasing Fibrillation Duration Enhances Relative Asymmetric Biphasic Versus Monophasic Defibrillator Waveform Efficacy", Circulation and Respiration, 376-384, Vol. 67 (1990); Bardy et al, "A Prospective Randomized Evaluation of Bipolar Versus Monopolar Waveform Pulses on Defibrillation Efficacy in Humans", Journal of the American College of Cardiology, 728-733, Vol. 14 (1989); and Winkle et al., "Improved Low Energy Defibrillation Efficacy in Man With the Use of a Biphasic Truncated Exponential Waveform", American Heart Journal, 122-127, Vol. 117 (1989). These articles describe a ratio of energy requirements of a biphasic waveform as compared to the monophasic waveform that is less than 1.0. None of these articles indicate that ratio may be dependent on waveform duration.
The present generation of defibrillators have, to a large extent, been designed using strength-duration data determined with monophasic waveforms. When monophasic truncated exponential waveforms are used, defibrillation waveforms with long pulse durations (&gt;10 msec) have no advantages over shorter duration waveforms.
Therefore, manufacturers of implantable defibrillator devices have used output capacitors with a capacitance of 60 to 150 microFarads and waveform durations less than 15 msec, and typically less than 10 msec. Waveform durations have been maintained substantially the same as manufacturers have changed their defibrillator designs to produce biphasic defibrillation shocks.
Recent patents further demonstrate that state-of-the-art defibrillation waveforms have durations shorter than 15 msec. See, for example, U.S. Pat. Nos. 4,548,203 to Tacker, Jr. et al., 4,953,551 to Mehra et al., and 4,821,723 to Baker et al.
It is known that the pulse duration has a major impact on the strength requirements for stimulating tissue. Since most theories of defibrillation involve stimulation of refractory tissue, one would expect the strength-duration relationship for defibrillation to be at least qualitatively similar to that of pacing. This has been born out by studies by Wassale et al. and Konig et al. and described in their respective articles "Bipolar Catheter Defibrillation in Dogs Using Trapezoidal Waveforms of Various Tilts", Journal of Electrocardiology, 359-366, Vol. 13 (1980), hereinafter "Wassale et al."; and "Amplitude-Duration Relationship for Direct Ventricular Defibrillation with Rectangular Current Pulses", Medical Biological Engineering, 388-395, Vol. 13 (1975), hereinafter "Konig et al.". The overall shape of the strength-duration curves for defibrillation with both monophasic truncated exponential and rectangular waveforms are similar to those determined for pacing. However, the pulse durations that minimize energy delivery requirements are lower for pacing (0.5 msec) as described by Irnich in the article "The Chronaxie Time and Its Practical Importance", PACE, 292-301, Vol. 3 (1980), hereinafter "Irnich", than for defibrillation (4 msec) as described by Wassale et al.
Defibrillation with monophasic waveforms appears to follow a hyperbolic strength-duration relationship, as does pacing. Defibrillation strength requirements have been measured for rectangular waveforms of different durations (1, 2, 5, 10, 20, 30 and 40 msec). See Konig et al. A good fit was found between their current amplitude data and a hyperbolic strength-duration curve with a chronaxie (a duration that gives minimum energy) at about 4 msec. In the aforementioned article by Wassale et al., strength-duration curves for monophasic truncated exponential waveforms were determined for several different tilts. Current requirements for these trapezoidal waveforms also increased as shock duration decreased, but shock energy was minimized at 2 to 4 msec, depending on the tilt. Both studies show a strength-duration relationship for defibrillation that is similar to that seen in pacing. However, it was found by Irnich that the chronaxie for pacing with rectangular waveforms is about 0.5 msec, while the chronaxie for defibrillation with monophasic waveforms is about 4 msec.
In an article by Chapman et al. entitled "Strength-Duration Curves of Fixed Pulse Width Variable Tilt Truncated Exponential Waveforms for Nonthoracotomy Internal Defibrillation in Dogs", PACE, 1045-1050, Vol. 11 (1988), the curve of delivered energy requirements versus pulse width for delivered energy is described for single-capacitor monophasic waveforms (exponential with fixed time constants). Energy requirements increased monotonically by only 50% as duration was increased from 2.5 to 15 msec, then increased dramatically for pulses of 20 msec durations. Energy requirements for the 20 msec pulse were 1.5 times that of the energy required for 15 msec pulses.
Feeser et al., in their article entitled "Strength-Duration and Probability of Success Curves for Defibrillation With Biphasic Waveforms", Circulation, 2128-2141, Vol. 82 (1990), showed that for at least some biphasic waveforms, extending waveform durations to more than 15 msec dramatically increases energy requirements. However, unlike our experiments described below, tilt was not held constant in any of their experiments. In the study described by Feeser et al., trailing edges for long waveforms were sometimes very low. MacDaniel et al., in their article entitled "Optimal Biphasic Duration for Canine Defibrillation with a `Single Capacitor` Waveform and a Non-Thoracotomy Electrode System", Annual Conference of IEEE EMBS, 1990, Volume 12, pages 636-637, showed that, for monophasic pulses of long duration, low trailing voltages are disadvantageous.
It has previously been shown that for fixed-duration shocks, the effectiveness of the truncated biphasic waveforms is critically dependent on the exact shape of the waveform. MacDaniel et al. and Feeser et al. have shown that for single capacitor waveforms, defibrillation shock efficacy varies as the ratio of the durations of the first and second phases is changed. Biphasic waveforms with first or second phases of short duration are actually less efficacious than monophasic pulses.
The strength-duration curves we found for the type of biphasic waveforms used are very different from those previously determined for truncated monophasic waveforms. The strength-duration relationship for energy requirements with biphasic truncated exponential waveforms were relatively flat and may greatly improve implantable defibrillator designs. Specifically, we discovered that defibrillation with long biphasic defibrillation pulses (20 msec) requires significantly lower peak voltages, with almost no increase in energy requirements.
Animal studies involving the present invention studied the effect of waveform duration on defibrillation efficacy of a biphasic truncated exponential waveform in six pigs. In each pig, defibrillation success curves were determined for waveform durations of 3, 5, 7, 10, 15 and 30 msec. The waveform shape chosen was of the "single capacitor" type with the trailing edge of the first pulse equal in magnitude to the leading edge of the second. The first pulse terminated at 40%, and the second pulse at 20% of the initial voltage of the first pulse, with the time constants of each phase being equal.
The effect of waveform duration on defibrillation efficacy of a monophasic truncated exponential waveform was determined in an additional six pigs. Waveform durations were the same as with the biphasic waveform, except the longest duration tested was 24 msec rather than 30 msec. The monophasic waveform terminated at 40% of the initial voltage.
All shocking electrodes were placed by fluoroscopy. One 11 F 3.4 cm long coil was positioned in the right ventricle and another coil of the same dimension was located in the high right atrium. A subcutaneous wire array electrode (100 cm.sup.2) was positioned such that the image of the right ventricular shocking coil and the subcutaneous electrode coincided in a lateral view. Thoracic X-ray films were made to verify correct placement of both shocking catheters and the subcutaneous electrode.
Fibrillation was allowed to run for 15 seconds after induction with a 9 volt DC signal between an endocardial sensing lead and an indifferent electrode. Then defibrillation was attempted with a defibrillator designed for research using a shock of the selected waveform duration. Rescue monophasic transthoracic defibrillation shocks were given with an external defibrillator (about 100 J) through an electrode pad on the right thorax and the subcutaneous electrode on the left. Only the initial shock of each fibrillation episode was used to evaluate defibrillation efficacy. The energy requirements for defibrillation were measured for the six waveform durations with the shocks being delivered according to an interleaved up/down protocol (described below). A total of 15 shocks were delivered by this protocol for each of the six durations (a total of 90 shocks). Data from these shocks were used to determine probability-of-success curves for each waveform duration.
TABLE I ______________________________________ Peak Current Amplitudes of First Shocks For Waveforms of Indicated Duration Biphasic Monophasic Duration Peak I (Amp) Duration Peak I (Amp) ______________________________________ 3 22.5 3 22.5 5 13.3 5 12.0 7 10.0 7 12.0 10 7.8 10 12.0 15 7.0 15 12.0 30 6.3 24 12.0 ______________________________________
Table I above shows the peak delivered current for the initial shock given for each of the six waveform durations. It should be noted that the current levels for first shocks were different for the monophasic and biphasic waveforms. For all succeeding shocks, the peak current and success of the immediately preceding shock with that waveform duration determined the peak current selected. If the prior shock at that waveform duration failed, then the peak current was increased by one step; for a successful shock, the peak current was decreased by one step. Initial current step sizes were 20%; at the first reversal, the step size was decreased to 10%.
The interleaving protocol was carried out as follows. The six durations were arranged in random order, then the first test shocks for each duration were delivered in that random order. The order was again randomized, and the second test shock for each duration were delivered according to the new order. That process was continued with 15 separate randomizations of order used, one for each of the 15 shocks delivered for each shock duration. This methodology ensured that, for example, the tenth shock given at one duration always preceded the eleventh shock given at another duration.
Pulse width and initial current were selected by the protocol described above. These values, plus the measured energy and voltage were recorded for each shock. Two minutes separated each fibrillation-defibrillation episode, unless more time was required for the animal to return to basal conditions as indicated by surface or endocardial ECG's, or blood pressure measurements.
For each pulse duration, two parameter probability-of-success curves (mid-point and width) were determined for delivered energy, peak voltage and current. To produce the sigmoid defibrillation success curves, modified error functions were fitted to the data of conversion success or failure for each measured value using the maximum likelihood method. For each pulse duration, the energies, voltages and currents at which the percent probabilities for defibrillation success equaled 50% (E.sub.50, V.sub.50 and I.sub.50) and 80% (E.sub.80, V.sub.80 and I.sub.80) were determined from the success curves.
As shown in FIG. 1, for the biphasic waveform, it was found that shock duration had no statistically significant effect on energy requirements over the range of 3 to 15 msec. Energy requirements were increased about 25% for shock durations of 30 msec. For the monophasic waveform as shown in FIG. 1, defibrillation energy requirements increased monotonically as shock durations were increased from 3 to 24 msec. The E.sub.50 for shocks 3 msec long was 11.6.+-.3.1 joules compared to 36.8.+-.9.7 joules for shocks 24 msec long.
For the biphasic waveforms, it was found that current and voltage requirements declined monotonically with increases in waveform duration as shown in FIG. 2. Peak current and voltage requirements were 2.5 times higher for waveform durations of 3 msec than for durations of 30 msec. For the monophasic waveforms, current and voltage requirements declined significantly as shock durations were increased from 3 to 10 msec as shown in FIG. 3. There was no significant further decreases as the durations were set at 15 and 24 msec, respectively.